The present invention refers to magneto-optical storage media for magneto-optically storing and reproducing information with a laser beam.
Magneto-optical storage media, an application of magneto-optical effects, are increasing their storage density as a result of a variety of research and development projects to develop repeatedly rewritable information storage media with a large capacity.
The magneto-optical storage medium has a short-coming that reproduction properties deteriorate with a relative decrease in the diameter or interval of storage bits, which form magnetic domains for storage, to the diameter of the light beam focused on the medium.
This is because the diameter of the light beam focused on a target storage bit encompasses an adjacent storage bit, and the information stored on the individual storage bits cannot be separately reproduced.
To eliminate the short-coming, attempts have been made to improve storage density through working on the arrangement and reproduction technique of the storage medium. One of the proposed methods is an expanded magnetic domain reproduction system by means of displacement of magnetic walls.
Here, a reference is made to a prior art, Japanese Laid-Open Patent Application No. 6-290496/1994 (Tokukaihei 6-290496 published on Oct. 18, 1994; hereinafter will be referred to as Prior Art 1) disclosing an expanded magnetic domain reproduction technology by means of displacement of magnetic walls.
According to the technology, in a magneto-optical storage medium, high density storage is realized using reproduction signals having an increased amplitude, by coupling magnetic films that form a multi-layered structure through an exchange force, and increasing tiny storage magnetic domains in a recording layer 104 in size by means of a magnetic domain expansion layer 101. FIG. 7(a) shows such an arrangement. Note that arrows are drawn in some layers to denote the directions of sub-lattice magnetization of the transition metals composing the layers, and also that magnetic walls (Bloch walls) 110 are formed in the layers between such adjacent magnetic domains that the directions of their magnetization are different from each other by 180xc2x0. The layers in which no arrows are drawn are non-magnetic. Portions of the magnetic layers in which arrows are absent denote loss of ordered magnetization in them due to temperature elevated to the Curie temperature or even higher.
There are four principal requirements for the magneto-optical storage medium as follows:
1. The recording layer 104 should be provided so as to stably hold tiny magnetic domains in place at temperatures ranging from room temperature to temperatures reached during reproduction.
2. The recording layer 104, the intermediate layer 102, and the magnetic domain expansion layer 101 should be coupled through an exchange force at least in a proximity of the Curie temperature, TC102, of the intermediate layer 102.
3. The intermediate layer 102 should lose ordered magnetization as its temperature rises past the Curie temperature TC102, cutting off the exchange coupling among the recording layer 104, the intermediate layer 102, and the magnetic domain expansion layer 101 above the Curie temperature TC102.
4. The magnetic domain expansion layer 101 should generate a low frictional force due to magnetic domain wall coercivity, and a temperature gradient should cause a magnetic wall energy gradient. Hence, the magnetic walls 110 move where the intermediate layer 102 functions so as to cut off the exchange coupling, with the portion to which magnetization is duplicated from a magnetic domain 104a in the recording layer 104 as an original. As a result, the magnetization in those regions become aligned to the same direction as that of the magnetic domain 104a. 
FIG. 7(b) is a graph illustrating the distribution of temperature in the middle of a track of a disk moving to the right relative to the person observing as a result of projection of a laser beam to the magneto-optical storage medium. Here, the disk is moving at such a high linear velocity that temperature is highest downstream of the center of the beam spot with respect to the direction of the movement of the beam spot.
FIG. 7(c) is a graph illustrating the distribution of the magnetic wall energy density xcex4101 in the magnetic domain expansion layer 101 in a circumferential direction. Typically, the magnetic wall energy density decreases with an increase in temperature, dropping to 0 above the Curie temperature. Therefore, when there is a temperature gradient in a circumferential direction as shown in FIG. 7(b), the magnetic wall energy density xcex4101 decreases with high temperatures as shown in FIG. 7(c).
The force, F101, exerted on the magnetic walls in the layers at position x along the circumference is given by the following expression:
F101=xe2x88x92dxcex4101/dx
The force F101 acts to move the magnetic walls to a lower magnetic wall energy level. The magnetic domain expansion layer 101, in comparison to the other magnetic layers, generates a low frictional force due to wall coercivity, i.e., is likely to allow movement of the magnetic walls. Therefore, when the exchange force is no longer available from the intermediate layer 102, the magnetic domain expansion layer 101 allows the force F101 to move the magnetic walls to a lower magnetic wall energy level.
In FIG. 7(a), prior to the projection of a laser beam to the disk, the three magnetic layers are coupled through an exchange force where temperature is equivalent to room temperature, while the magnetic domains stored in the recording layer 104 have been duplicated to the magnetic domain expansion layer 101. Here, in each of the layers, there exist magnetic walls between such adjacent magnetic domains that have mutually reverse magnetization directions.
Where temperature has been raised to the Curie temperature, TC102, of the intermediate layer 102 or higher, the intermediate layer 102 loses magnetization, cutting off the exchange coupling between the magnetic domain expansion layer 101 and the recording layer 104; therefore the magnetic domain expansion layer 101 can no longer hold the magnetic walls in place, allowing the magnetic walls to move toward a higher temperature portion according to the force F101 exerted on the magnetic walls. Here, the magnetic walls move at a velocity sufficiently faster than does the medium. Therefore, the duplicate magnetic domains in the magnetic domain expansion layer 101 are larger in size than those stored in the recording layer 104.
However, the medium described in Prior Art 1 entails following problems: since the exchange coupling from the recording layer 104 through the magnetic domain expansion layer 101 is cut off where temperature has risen to the Curie temperature, TC102, of the intermediate layer 102 or higher, the magnetic walls become movable in the magnetic domain expansion layer 101, whereas a parasitic magnetic field generated by the storage magnetic domains of the recording layer 104 builds up an unignorable magnetostatic coupling force.
The magnetostatic force arising from the magnetic fields generated by the other magnetic layers and the like, as well as that arising from the magnetic moments of those magnetic layers per se, is ignorably small in comparison to the exchange force, since an exchange force arises from exchange of electrons between magnetic layers at their interface. However, when the exchange coupling is cut off as in the above case, the magnetostatic coupling force is no longer ignorable. According to a super-resolution technology disclosed in Japanese Laid-Open Patent Application No. 10-40600/1998 (Tokukaihei 10-40600; published on Feb. 13, 1998), Japanese Laid-Open Patent Application No. 6-150418/1994 (Tokukaihei 6-150418; published on May 31, 1994), and other documents, the magnetization direction of the reproduction layer is caused to conform to the magnetization direction of the recording layer by the use of magnetostatic coupling with the recording layer.
In other words, in the medium arrangement described in Prior Art 1, a magnetostatic coupling force arises from the magnetic field generated by the recording layer 104 and exerted on the magnetic domain expansion layer 101 so that the magnetization direction of the magnetic domain expansion layer 101 aligns to the magnetization direction of the recording layer, interrupting movement of the magnetic walls in the magnetic domain expansion layer 101, presenting an obstacle in the expansion of the magnetic domains. An embodiment of Prior Art 1 suggests, as a solution, a method to attenuate the generated magnetic field by fabricating the magnetic layers from compensating compositions.
However, according to the method, since temperature in the beam spot does not have a uniform distribution, resulting in an uneven distribution of magnetization; consequently, a magnetic field is generated by the recording layer at least in some regions in the beam spot. In addition, in the embodiment of Prior Art 1, the intermediate layer 102, interposed between the recording layer 104 and the magnetic domain expansion layer 101, is relatively thin at about 10 nm. Therefore, the, magnetic domain expansion layer 101 is acted upon by the magnetic field generated by the recording layer 104 at a very short distance. The closer the magnetic domain expansion layer 101 is to the recording layer 104, the greater magnetostatic coupling force the magnetic domain expansion layer 101 experiences; adverse effects of the force, as a result, become increasingly unignorable, interrupting the movement of the magnetic walls in the magnetic domain expansion layer 101.
The present invention has an object to offer a magneto-optical storage medium that can suppress magnetic fields generated by a recording layer, realize satisfactory movement of magnetic walls and expansion of magnetic domains, and increase signal strength.
A magneto-optical storage medium in accordance with the present invention, in order to achieve the above object includes:
a recording layer in which a plurality of storage magnetic domains are formed;
a first intermediate layer for cutting off exchange coupling with the recording layer when temperature rises past a predetermined temperature;
a magnetic domain expansion layer in which magnetic walls move toward a higher temperature portion so as to form expanded magnetic domains when the first intermediate layer cuts off the exchange coupling with the recording layer; and
a magnetic masking layer, provided between the recording layer and the first intermediate layer, that is coupled with the recording layer through an exchange force at temperatures that are not higher than the predetermined temperature, and meanwhile cuts off magnetostatic coupling of the magnetic domain expansion layer with the recording layer at temperatures that are higher than the predetermined temperature.
With the arrangement, when temperature does not exceed the predetermined temperature, the recording layer, the magnetic masking layer, the first intermediate layer, and the magnetic domain expansion layer are coupled with each other through an exchange force, allowing the storage magnetic domains in the recording layer to be duplicated to the magnetic domain expansion layer. Meanwhile, as temperature rises past the predetermined temperature, the first intermediate layer cuts off the exchange coupling between the recording layer and the magnetic domain expansion layer, causing the magnetic domain expansion layer to lose magnetic order. As a result, the magnetic domain expansion layer can no longer hold the magnetic walls in place, allowing the magnetic walls to move toward a higher temperature portion with those magnetic domains that have been duplicated by the exchange coupling as an original. Hence, the duplicate magnetic domains expand toward a higher temperature portion, forming expanded magnetic domains in the magnetic domain expansion layer.
As temperature rises past the predetermined temperature, besides that the first intermediate layer cuts off the exchange coupling between the recording layer and the magnetic domain expansion layer as above, the magnetic masking layer cuts off the magnetostatic coupling between the recording layer and the magnetic domain expansion layer. This prevents the magnetic field (parasitic magnetic field) generated by the recording layer from acting on the magnetic domain expansion layer and thus presenting an obstacle to expansion of the magnetic domains at temperatures that are higher than the predetermined temperature; therefore, in the magnetic domain expansion layer, the magnetic walls move in a satisfactory fashion, forming expanded magnetic domains accurately and precisely.
The formation of the expanded magnetic domains increases reproduction signal strength; reproduction signals are now obtainable with a sufficiently large amplitude without being adversely affected by noise even if linear storage density is increased. As a result, the reliability of a reproduction operation increases dramatically.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.